In pro and semi pro RC systems the servos and speedos (motor speed/direction controllers) are commanded by 5v pwm signals. Only three wires are used, two for power supply and one for signal. Where high loads are used, there are additional power supply lines, but their control is the same. The receiver is generating these signals (according to the settings on the transmitter), you can plug as many servos/speedos/etc, into the receiver as many channels the system has. With cars two channels are enough, one for steering and one for throttle/brake.
The method for transmitting the radio signal itself may be different on the am and fm bands, nowadays (for 4-5 years now) a digital method is used on 2.4ghz (for all radios), but the protocol may be different for each manufacturer.